I've never heard of a gunmaker checking to see if the front lump and inside floor have an exceptionally tight fit - or of anyone caring about it.
Perhaps they did/do, but it sounds odd.
Not having a through-lump should make the bbls easier to fit and require less time overall.
When you put the bbls back on face, there's less work to be done, too. And there's no ugly gap on the through lump like you see here:https://www.gunsinternational.com/guns-f...un_id=101130969
The strength argument is odd, too.
Scott used through-lumps on their sidelocks from the 1870s to at least the 1930s.
I've seen them on old, rising-bite .450NE & .470NE Rigby sidelock DRs & 12g shotguns, too. I'm pretty sure Rigby is using them today on their new rising-bite shotguns and DRs.
So if through-lumps weaken actions, why?
I suspect through lumps were more of a PITA overall. I bet makers got rid of them because of the extra work they required and to tidy up/modernize the look of their guns.